1. Field of the Invention
The present invention generally relates to a wireless electric power transmitter, and more particularly to a wireless electric power transmitter for transmitting electric power to a wireless electric power receiver.
2. Description of the Related Art
Mobile terminals such as a mobile phone, a Personal Digital Assistant (PDA) and the like are driven with rechargeable batteries due to their nature, and the battery of the mobile terminal is charged through supplied electric energy by using a separate charging apparatus. In general, a separate contact terminal is arranged outside of the charging apparatus and the battery, and the charging apparatus and the battery are electrically connected to each other through contact between their contact terminals.
However, since the contact terminal outwardly protrudes in such a contact type charging scheme, the contact terminal is easily contaminated by foreign substances and thus the battery charging may not be correctly performed. Further, the battery charging may also not be correctly performed in a case where the contact terminal is exposed to moisture.
Recently, a wireless charging or a non-contact charging technology has been developed and used for electronic devices to solve the above-mentioned problem.
Such a wireless charging technology employs wireless electric power transmission/reception, and corresponds to, for example, a system in which a battery can be automatically charged when a portable phone is not connected to a separate charging apparatus, but merely placed on a charging pad. Accordingly, a waterproof function can be improved since electronic products are wirelessly charged through the wireless charging technology, and the portability of electronic devices can be increased since there is no need to provide a wired charging apparatus.
The wireless charging technology largely includes an electromagnetic induction scheme using a coil, a resonance scheme using a resonance, and an RF/microwave radiation scheme converting electrical energy to microwaves and then transmitting the microwaves.
It is considered up to now that the electromagnetic induction scheme is mainstream, but it is expected that the day will come when all electronic products are wirelessly charged, anytime and anywhere, without a wire in the near future on the strength of recent successful experiments for wirelessly transmitting power to a destination spaced away by dozens of meters through the use of microwaves.
A power transmission method using the electromagnetic induction corresponds to a scheme of transmitting electric power between a first coil and a second coil. When a magnet is moved in the coil, induced current is generated. Accordingly, a transmission side generates a magnetic field by using the induced current and a reception side generates energy through an induced current according to changes in the magnetic field. This phenomenon is referred to as the magnetic induction, and the electric power transmission method using magnetic induction has high energy transmission efficiency.
With respect to the resonance scheme, a wireless charging system employs a concept in physics whereby resonance is the tendency in which when a tuning fork oscillates at a particular frequency, a wine glass next to the tuning fork oscillates at the same frequency. In the resonance scheme an electromagnetic wave containing electrical energy resonates instead of sounds resonating. The resonated electrical energy is directly transferred only when there is a device having a resonance frequency nearby, and parts of electrical energy which are not used are reabsorbed into an electromagnetic field instead of being spread in the air, so that the electrical energy does not affect surrounding machines or people, unlike other electromagnetic waves.
Meanwhile, a conventional wireless electric power transmitter generally includes a class E type amplifier. However, the class E type amplifier has a problem in that it is difficult to maintain an output with a desired magnitude in a circumstance similar to a resonance scheme in which load impedance is changed. Accordingly, the class E type amplifier changes a value of a required device as the load impedance is changed. On the other hand, a class D type amplifier does not change a value of a required device as load resistance is changed. However, since the conventional class D type amplifier has a problem in that output electric power reduces as the load resistance increases, there is a problem in that it is not suitable for use in the wireless electric power transmitter.